Field of the Invention
The present invention relates to the determination of thresholds for a method for detecting defective printing nozzles.
The technical field of the invention is the field of digital printing.
In digital printing, namely in inkjet printing, the malfunctioning of individual printing nozzles of the print head of the inkjet printing machine is a common phenomenon. Nozzle malfunctions can take many forms: a nozzle may print at a reduced ink drop volume; the print dot of the printing nozzle may deviate, i.e. the nozzle prints at an angle; a nozzle may fail completely. Foreign bodies, in particular dust particles, that have entered the printing nozzle or hardened ink residues in a printing nozzle of the print head are examples of common causes of such malfunctions. All these different types of malfunctions of defective printing nozzles are referred to by the generic term of “missing nozzle”. Such missing nozzles result in specific defects in the print to be created. A failed printing nozzle for instance causes a line-shaped artifact because no ink is applied at the respective location. In a monochrome print, what is known as a “white line” is created at the location of the defective printing nozzle because the printing substrate, which is white in most cases, shines through. In a multicolor print, where the inkjet printing machine prints multiple colors on top of one another to obtain a specific color value, the target color value is distorted because the failed printing nozzle does not contribute its designated color proportion. Printing nozzles that print with a reduced ink volume have a similar effect. Printing nozzles that print with a large angular offset create an additional problem: In addition to a white line that is created because the printing nozzle does not print at the designated location, a black line is created because the large angular offset causes the printing nozzle to print at a location that already receives ink from another printing nozzle. Due to the increased amount of ink that is applied at this location, a line-shaped artifact is created whose color value is higher than actually intended. This is referred to as a “black line”.
To minimize the effects that such printing nozzle malfunctions have on the quality of the print, various methods for compensating for such defective printing nozzles have been applied. However, the first step necessary to be able to take compensatory steps, is to accurately identify the defective printing nozzle. Different approaches have become known in the art to detect such defective printing nozzles. One known approach, for instance, is to provide an image sensor for recording the print that has been created by the inkjet printing and to compare the print that has been digitized in this way to an reference image to be able to detect deviations that may be caused by defective printing nozzles. However, this approach, which is mostly carried out in an automated quality control process, suffers from a variety of problems. For instance, it allows only those printing nozzles to be monitored that actually contribute to the creation of the respective print. Thus printing nozzles that are not needed for a current print job cannot be monitored to find malfunctions. In addition, in many cases, the print image data that are to be created in preparation of the print job are unsuitable for an accurate functional check of an individual printing nozzle. Another problem is the allocation of an image defect in the recorded print to a specific printing nozzle. Due to restrictions of the image recording system such as the resolution of the image sensor that is used, such an allocation—albeit essential for a correct functional monitoring of the individual printing nozzles—is only possible to a limited extent.
A common method for detecting defective printing nozzles is to print what are referred to as nozzle test charts, which are placed and printed onto the printing substrate outside the actual print. Such nozzle test charts are recorded by the image recording system and evaluated. Since the nozzle test chart has been configured with the specific intention of allocating a specific part of the test chart to every printing nozzle, an evaluation of the recorded nozzle test chart provides unequivocal information on the functioning of all contributing printing nozzles. The evaluation is made in a computer-assisted way and is usually implemented by the computer of the respective image recording system. However, it is possible to forward the data to a specific evaluation computer. The known nozzle test charts themselves vary considerably. One chart known in the art consists of a vertical line printed by every printing nozzle. Since the resolution of the image sensor that records the nozzle test chart is often lower than the resolution of the print head, the nozzle test chart is mostly arranged in a way that only every xth nozzle in a row of adjacent nozzles on the print head prints a vertical line rather than every printing nozzle in a row. Subsequently, every (x+1)th printing nozzle of the row underneath prints a vertical line and so on until all printing nozzles that need to be tested on the print head have printed their respective vertical line. Due to the countability and unambiguousness of individual vertical lines, every single line may be allocated to a specific printing nozzle. For the evaluation, conclusions on the status of the printing nozzle in question may be drawn from parameters such as the degree of deviation of the line from the known target position thereof or the continuity of the printed line. A disadvantage of this approach is, however, that it is difficult to correlate the degree of deviation of the printing nozzle from the target position and the extent to which the nozzle will be responsible for a typographic defect in the final print if at all. Thresholds for evaluating whether the printing nozzle prints in an acceptable functional range or needs to be classified as defective are used for this purpose. If a threshold for evaluating whether a printing nozzle is defective or not is set to be too sensitive, many errors of judgment may be the consequence, i.e. printing nozzles that operate correctly and have a small deviation but are still suitable for printing would be recognized as defective and would later be compensated for. Yet printing nozzle compensations will always result in lower print quality in the print to be created than a print that is created with a complete set of functioning printing nozzles. If, on the other hand, a threshold is too lax, the nozzles that are typographically problematic and cause defects in the print are not recognized, remain uncompensated for, and continue to create defects in the print.
The defined threshold may be a constant value. However, an expedient threshold depends on the current printing conditions such as the ink flow behavior, which in turn depends on the substrate that is printed on and on the ink dryer settings, for instance. In addition, the measuring system that records the nozzle chart (camera system) may create measuring noise, which applies an error to a theoretically assumed value of the threshold (e.g. a deviation in the x direction by one half of the width of the nozzle writing range). Thus a definition of a constant value is difficult both from the measurement technology perspective and from the perspective of varying printing conditions.
A statistical value derived from the measured values of all nozzles may be taken as an alternative threshold. This may be n times the standard deviation of the deviation of the nozzle from the target position in the x direction, for instance. Such a threshold causes nozzles that clearly print in a way that is different from the other nozzles to be classified as problematic. A nozzle may for instance be classified as problematic if the deviation from the target position is greater than 4 times the standard deviation of all deviations of all nozzles from the target position in the x direction. A disadvantage of this process is that it assumes a functioning set of nozzles in which all nozzles that have values below the criterion of n times the standard deviation do not cause any typographic defects under the current printing conditions. Yet if many nozzles of the set no longer function because of a considerable localized contamination, the threshold defined as n times the standard deviation will be higher than the values of many nozzles that no longer function. These nozzles will then not be recognized as problematic.
Thus it is known in the art to print area coverage elements instead of nozzle test charts. In such a case, all contributing printing nozzles print a halftone or solid area for test purposes. Then the image is recorded to check whether the area coverage element that has been printed contains image artifacts such as white lines, black lines, or the like that indicate printing nozzles that do not function correctly. This approach is very useful for a general detection of printing nozzles that cause problems in the print. But as it is the case with the detection on the basis of the actual printed image, again this process is faced with the inherent problem that the individual printing nozzles that cause these defects cannot be identified within the area coverage element. It is only possible to identify the region in which the defective printing nozzle must be located but not the individual specific printing nozzle itself that is defective. The latter would only be possible if a high-performance image recording hardware of high image recording resolution was provided. Even then, due to the ink flow behavior, it may only be the defect that is identifiable. In this case it remains impossible to identify the specific nozzle because there is no unequivocal correlation between the visible defect in the area and a specific nozzle. In a similar way, the failure of a nozzle pair or of special nozzles in a neighborhood range may only be detected by means of a camera of extremely high resolution, if they are not altogether impossible to detect.
Published, non-prosecuted German patent application DE 10 2016 224 303.9, which has not yet been published, discloses to print the area coverage elements in addition to the nozzle test chart with multiple different area densities. If a deviating nozzle is found in the course of the evaluation of the nozzle test chart, the corresponding position in the area coverage elements of different area densities may be checked to see whether the specific defective printing nozzle causes print defects and if so at what area densities. Compensatory measures will then be taken only for area densities at which the defective printing nozzle causes defects in the print. However, a disadvantage of this approach is that for an accurate assessment and categorization of a printing nozzle with print deviations that has been detected in the nozzle test chart, the area coverage element with the various area densities always needs to be printed onto the printing substrate. Since the image is continuously recorded during the production run on the inkjet printing machine for quality control purposes including the detection of defective printing nozzles, this means that the nozzle test chart and in this case the area coverage element at multiple area densities need to be printed onto every xth print sheet. This means that a considerably increased effort is required for the entire detection process. For it is not only the nozzle test chart that needs to be evaluated but also the area coverage element with multiple area densities, and both results continuously need to be compared to one another. In addition, this method of the prior art does not give any information on how to solve the problem of determining accurate thresholds for evaluating every single nozzle that exhibits deviations in the nozzle test chart.
Another document of the prior art is European patent application EP 25 05 364 A2, corresponding to U.S. Pat. No. 8,646,869, which discloses a method for determining printing nozzles that exhibit print deviations and involves the definition of thresholds for assessing when a printing nozzle exhibits print deviations. This method of the prior art does not disclose the printing of an area coverage element. Instead, the thresholds are defined exclusively from the detection and evaluation of printed nozzle test charts. Thus these methods continue to suffer from the disadvantage that the thresholds for evaluating a deviation of the printing nozzle are defined without considering the actual print result. This means that due to potentially erroneous thresholds, these methods likewise run the risk of detecting deviations that do not actually create any visible print defects and consequently do not affect the print quality of the printed product to be created.